The high-level architecture of modern telecommunications systems such as the Universal Mobile Telecommunications System (UMTS) typically comprises three different types of network components: Radio Access Networks (RANs), Core Networks (CNs) and User Equipment (UE) as shown in FIG. 1. The CN is responsible for switching and routing calls and data connections to and from external networks such as wire-based telephone networks or the Internet. The RAN is located between the CN and the UE and handles all radio-related functionality. The UE constitutes the interface between the user and the RAN.
A modern CN comprises dedicated nodes for providing CS services on the one hand and PS services on the other hand. CS services are typically provided via a CN node called Mobile Services Switching Centre/Visitor Location Register (MSC/VLR). The part of the CN that is accessed via the MSC/VLR is often referred to as the CS domain. PS services are generally provided via a Serving General Packet Radio Service Support Node (SGSN), and the part of the CN that is accessed via the SGSN is called the PS domain. In conventional telecommunications networks, the CN serves a particular UE often in the CS and PS domains simultaneously.
In recent years, network sharing has become a vehicle for using valuable network resources more efficiently. In one network sharing scenario called Multi-Operator Core Network (MOCN), multiple CNs typically belonging to different operators share a common RAN. In the exemplary MOCN scenario shown in FIG. 1, three CNs (CN A, CN B and CN C) belonging to different CN operators are attached via a so-called Iu interface to a single RAN.
In networks of the MOCN type, it is desirable that a particular UE will be registered in the CS domain (MSC/VLR) and the PS domain (SGSN) of one and the same CN operator. With reference to FIG. 1, it should thus for example be avoided that a particular UE is connected to the MSC/VLR of CN A and to the SGSN of CN B. Such a connection situation will not occur if the UE supports network sharing (“supporting UE”). A supporting UE is configured to select a particular CN as its “serving CN” in the shared network and to signal this selection to the RAN to which the UE is attached. The RAN will then automatically route the PS and CS registration for this UE to the signalled CN.
The supporting UE bases its CN selection on additionally broadcast MOCN system information. However, a UE not supporting network sharing (“non-supporting UE”) will simply ignore such additional system information and will thus leave CN selection to its RAN. Conventionally, a RAN bases its CN selection on a Network Resource Indicator (NRI) initially received from the UE that requires registration in the PS and CS domains. The NRI is in most cases indicative of a non-permanent and/or local identity of the terminal. For example, the NRI may be indicative of the Temporary Mobile Subscriber Identity (TMSI) as assigned in the CS domain, or of the P-TMSI as assigned in the PS domain.
Since the RAN does not have any knowledge of how the TMSI and P-TMSI values were assigned in the respective domains, it may send the CS registration message and the PS registration message for a particular UE to two different CNs. If each of the associated CN operators has a roaming agreement with the user of the UE, both CNs will accept the registration of the UE and will send their associated network domain identifier, via the RAN, to the UE. The UE thus receives different network domain identifiers for the CS and PS domains, and faces an inconsistent situation that could give rise to malfunctions.
In order to guarantee in a MOCN scenario that a non-supporting UE is served by one and the same CN operator in both domains, it has been proposed to use an optional interface (the so-called Gs interface) between the MSC/VLR and the SGSN for coordinating UE registrations in the CS and PS domains (see, e.g., section 4.2.5 of Technical Specification 25.251 of the Third Generation Partnership Project 3GPP). However, the optional Gs interface is currently not implemented in all of the CN nodes concerned. Moreover, implementation of the Gs interface is not only costly, but additionally adds a layer of complexity to the CN design.
Accordingly, there is a need for an improved technique that allows for coordinated CS and PS domain registrations.